De-Excitation of ZnS and ZnCdS Phosphors by Electric Fields

Abstract

The change of impedance (photoconductivity) of certain ZnS and ZnCdS powder phosphor samples, brought about by external ac (60 cps) and dc fields (up to 15 000 volts/cm), applied both during and after excitation with ultraviolet light (3660 A), is described and compared with impedance changes due to irradiation with infrared light. Application of a dc field simultaneously with the exciting radiation results first in a sudden drop in both the capacitance ($\frac{\Delta C}{C_0}$) and the dissipation factor ($D=\frac{1}{Q}$) followed by a recovery to a new equilibrium level below the field-free case. Removal of the field is accompanied by another drop and a recovery to the original field-free values. The partial recovery is not seen during ac field application under the same conditions but the complete recovery also occurs upon field removal. Applied during the photoconductivity decay, both ac and dc fields accelerate the decay. The sudden drop and partial recovery under dc fields is also seen during the photoconductivity decay but to a lesser extent.